P&F's strength was its ability to take a very simple formula to secure the structure, but build imaginative twists and jokes on top of it. They rarely ventured outside the "Boys build the thing, candace tries to bust, perry foils doof's plan" cycle. When they did, it was always well thought out and planned.
With Milo, they didn't have that formula, nor that structure, you you were just left with a disorganized mess of different plot points.
Oh, also the characters are way, way weaker.